An analysis is made of natural convection mass transfer in a ternary gas mixture adjacent to a vertical plate. The main feature of the analysis is the proper mathematical description of the buoyancy force and of the boundary conditions. With regard to the latter, account is taken of the selective impermeability of the plate to certain components of the mixture. The governing boundary layer equations are shown to admit a similarity solution, and a numerical scheme for solving the similarity equations and boundary conditions is outlined. The general formulation is specialized to the case of sublimation from a solid, vertically oriented naphthalene plate situated in a humid air environment. Solutions were obtained for temperature levels corresponding to those encountered in practice and for relative humidities of water vapor between zero and one. It was found that the mass transfer results were unaffected by the presence of humidity, and this finding gives license to the utilization of the naphthalene technique without special concern about the humidity level. In addition, the transverse velocity at the plate surface associated with the mass transfer was too small to affect the magnitude of the mass transfer.